1. Field of the Invention
The present invention relates to positron emission tomography (PET). More specifically, it relates to radiopharmaceuticals used in PET. More particularly, it relates to formulations and methods for stabilizing these radiopharmaceuticals. These formulations and methods prevent these radiopharmaceuticals from degrading due to many factors including radiolysis.
2. Description of Related Art
Over the past 10 years positron emission tomography (PET) has evolved from a research tool to a commonly applied clinical diagnostic test. At the heart of PET imaging are isotope-labeled radiopharmaceuticals that undergo specific biological transformations (e.g. enzymatic transformation, such as phosphorylation) or bind to biomolecules with high specificity and affinity. PET imaging not only enables disease diagnosis in a clinical setting, but also supports the development of new therapeutic drugs by allowing receptor occupancy and pharmacokinetic properties to be evaluated in vivo.
The formulation and stabilization of radiopharmaceuticals for PET imaging is a critical component to the manufacturing process. Radiopharmaceuticals must be formulated appropriately for human dosing; the most common delivery route being intravenous administration of aqueous solutions. At a minimum, the formulation must not adversely compromise the stability of the radiopharmaceutical for the duration of its shelf-life. In an ideal scenario, the formulation provides an extra measure of protection against radiopharmaceutical degradation. This aspect of stability is critical since bulk dose vials containing radiopharmaceutical are often made in high strength (mCi/mL) to enable dispensing of multiple doses over a period of several hours. In addition, it is critical to maintain a high radiochemical purity to achieve the best image quality possible. If the stability of the bulk vial is compromised during the duration of its shelf-life, the dose may be unusable and unfit for human dosing or imaging.
Radiopharmaceuticals can experience instability as a function of strength (mCi/mL), pH, temperature and specific activity. One of the major issues with respect to radiopharmaceutical formulations is radiolysis (i.e. radiolytic degradation), which can occur while the radiopharmaceutical is aging in the dosing solution or bulk vial. The radiolysis process is not fully understood but current research suggests that ionizing radiation, generated via positron decay, induces the formation of radicals. See Jan Van Den Bos, (Healthcare, G., Ed.) (2009) (related to WO 2009/059977); Maxim Y. Kiselev et al., (Isotopes, E., Ed.) (2004) (related to WO 2004/043497; Jianqing Chen et al., (S.P.A., B. I., Ed.) (2005) (related to WO 2005/009393); Richard M. Fawdry, Radiolysis of 2-[18F]fluoro-2-deoxy-D-glucose (FDG) and the Role of Reductant Stabilizers, Applied Radiation and Isotopes, vol. 65, pp. 1193-1201 (2007).
For example, one reference describes radiolysis as being “caused mainly by oxidation by free radicals that are produced by the interaction of ionizing radiation from the 18F-isotope with the water solvent and possibly air”. Maxim Y. Kiselev et al. (Isotopes, E., Ed.) (2004). Another reference cites that “the interaction of ionizing radiation with dissolved oxygen (O2) can generate very reactive species such as superoxide radicals. These radicals are very reactive towards organic molecules”. Jianqing Chen et al. (S.P.A., B. I., Ed.) (2005). As a consequence of the formation of these radicals, they can react further with each other, other radicals, oxygen and/or the radiopharmaceutical itself eventually causing radiolytic decomposition of the radiopharmaceutical.
Experiments using ionizing radiation on solutions of thymdine demonstrate the destructive effects caused by the decomposition products that react with thymidine. See J. R. Wagner et al., Photo and Radiation-Induced Formation of Thymidine Hydroperoxides, Bioelectrochemistry and Bioenergetics, vol. 18, pp. 155-162 (1987); R. Teoule & J. Cadet, Comparison of Radiolysis Products of Thymine and Thymidine with E.S.R. Results, Int'l J. Radiation Biology, vol. 27, pp. 211-222 (1975). The effects are further exacerbated by the presence of oxygen and often lead to peroxide-containing adducts. While these reports utilize an external source of radiation, the proposed decomposition adducts react with thymidine in a fashion consistent with radiolysis. For example, the author states the following:                “The formation of hydroperoxides during gamma radiolysis of dThd in oxygenated solutions most likely under these conditions involves the initial reactions of hydroxyl radicals with dThd. Hydroxyl radicals react with dThd by addition to the 5,6 double bond and by hydrogen abstraction either from the sugar moiety or from the methyl group.”See J. R. Wagner et al., Photo and Radiation-Induced Formation of Thymidine Hydroperoxides, Bioelectrochemistry and Bioenergetics, vol. 18, pp. 155-162 (1987).        
It is clear that ionization radiation, albeit from an external source, can lead to radiolysis on non-radioactive species, especially in the presence of air (oxygen).
The byproducts from the radiolysis-induced radical reactions are believed to be strongly oxidizing. For example, if hydroxyl radical is formed during radiolysis, it may combine with another hydroxyl radical to form hydrogen peroxide, a strong oxidizer. In another example, it is widely believed that ionizing radiation in the presence of oxygen leads to the formation of superoxide, a highly oxidizing and reactive species, which rapidly degrades radiopharmaceuticals. In an effort to combat the negative effect of these radicals in a strongly oxidizing environment, stabilizers are often added to the dosing solution. More specifically, these stabilizers are comprised of radical scavengers and/or anti-oxidants (i.e. reductants), both of which are believed to exert a protective effect upon the radiopharmaceutical against radiolysis. The effect of reductants on the inhibition of [F-18]FDG radiolysis is well studied in the art. See Richard M. Fawdry, Radiolysis of 2-[18F]fluoro-2-deoxy-D-glucose (FDG) and the Role of Reductant Stabilizers, Applied Radiation and Isotopes, vol. 65, pp. 1193-1201 (2007).
Table 1 shows some commonly-used tracers and stabilizers commonly-used with these tracers.
TABLE 1Common stabilizers for [F-18]-labeled radiopharmaceuticalsStabilizerTracerReferenceEthanol[F-18]FDGWO2004043497N-t-butyl-alpha-[F-18]AV-19Appl. Radiat. Isot. 2009,phenylnitrone (PBN)67, 88-94Sodium ascorbate[F-18]AV-19Appl. Radiat. Isot. 2009,67, 88-94Ascorbic acid[F-18]AV-45WO2010078370Ascorbic acid[F-18]FDDNPAppl. Radiat. Isot. 2008,66, 203-207Gentisic acid[F-18]FDGWO2009059977Calcium chloride2-[F-18]fluoromethyl-Nucl. Med. Biol. 2008,L-phenylalanine35(4), 425-432
Consistent with the thinking that oxidation plays a negative role in radiopharmaceutical stabilization; oxidants are not used to stabilize radiopharmaceuticals. Oxidants are expected to contribute to further radical growth, and thus increase the propensity for radiolysis. One reference discloses that stabilized radiopharmaceutical compositions are defined as those which are preferably stored in an environment from which oxygen gas has been removed. Jan Van Den Bos, (Healthcare, G., Ed.) (2009).
The above stabilizers are used in formulations that are devoid of oxygen. Since the stabilizers are largely anti-oxidants, using them to stabilize radiopharmaceuticals in the presence of oxygen would be expected to lessen their protective effect. For example, ascorbic acid rapidly degrades in the presence of oxygen, often changing color as a result of this degradation. The rapid decay of these stabilizers in air also means that solutions containing these stabilizers cannot be stored for long periods of time.
Accordingly, if oxygen was found to have a protective effect against radiolysis, then the addition of oxygen and certain non-oxidizing excipients into radiopharmaceutical formulations may have a synergistic effect that could not be accomplished through the use of either oxygen or the excipient alone. For example, if a non-oxidizing excipient such as maleic acid (MA) exerted a stabilizing effect on radiopharmaceuticals in the absence of oxygen, MA's protective effect on the radiopharmaceutical stability profile could be substantially increased in the presence of oxygen.
Maleic acid (MA) is a dicarboxylic acid that is a common excipient in non-radioactive injectable formulations. Its toxicity profile is well known. Int'l J. Toxicology, Am. C. Toxicology, Final Report on the Safety Assessment of Maleic Acid, vol. 26, suppl. 2, pp. 125-130 (2007). It is listed as an inactive ingredient for injection by the United States Food and Drug Administration (FDA) with a maximum potency of 0.01%.
MA causes a reversible malfunctioning of the proximal renal tubes in kidneys by forcing materials intended for re-absorption (glucose, HCO3−, etc.) to be excreted into the urine. Somchai Eiam-ong et al., Insights Into the Biochemical Mechanism of Maleic Acid-Induced Fanconi Syndrome, Kidney Int'l, vol. 48, pp. 1542-1548 (1995); Edgar J. Rolleman et al., Kidney Protection During Peptide Receptor Radionuclide Therapy with Somatostatin Analogues, Eur. J. Nuclear Med. & Molecular Imaging, vol. 37, pp. 1018-1031 (2010); Salim K. Mujais, “Maleic Acid-Induced Proximal Tubulopathy: Na:K Pump Inhibition”, J. Am. Soc'y Nephrology, vol. 4, no. 2, pp. 142-147 (1993). The effect MA exerts on the kidneys mimics the human disease known as Fanconi Syndrome. The proposed mechanism of action of maleic acid's effect is to a) cause direct inhibition of proximal tubule Na-K-ATPase activity and b) force membrane-bound phosphorus depletion. H. Al-Bander et al., Phosphate Loading Attenuates Renal Tubular Dysfunction Induced by Maleic Acid In the Dog, Am. J. Physiology, vol. 248, pp. F513-F521 (1985). In dogs, this effect is seen when administered at 20 mg/kg (440 mg total dose). The human dose equivalent for this effect is predicted to be approximately 15 mg/kg (1000 mg total dose). In rats, the effect is seen when administered at 50 mg/kg (12.5 mg total dose). The human dose equivalent for this effect is predicted to be approximately 12.3 mg/kg (855 mg total dose).
MA is commonly used in non-radioactive injectable solutions (antihistamine, utertonic, chemotherapeutic) as a counter salt or to modulate pH of the injectable dose. See H. G. Boxenbaum et al., Pharmacokinetic and Biopharmaceutic Profile of Chlordiazepoxide HCl In Healthy Subjects: Single-Dose Studies by the Intravenous, Intramuscular, and Oral Routes, J. Pharmacokinetics & Biopharmaceuticals, vol. 5, no. 1, pp. 3-23 (1977); E. A. Peets et al., Metabolism of Chlorpheniramine Maleate In Man, J. Pharmacology & Experimental Therapeutics, vol. 180, pp. 364-374 (1972); R. G. Strickley et al., Solubilizing Excipients In Oral and Injectable Formulations, Pharmaceutical Research, vol. 21, no. 2, pp. 201-230 (2004); J. Verweij et al., “Frequent Administration of Dabis Maleate, a Phase I Study”, Annals of Oncology, vol. 3, pp 241-242 (1992); William Sacks, Evidence For the Metabolism of Maleic Acid In Dogs and Human Beings, Science, vol. 127, p. 594 (1958); G. Tagliabue et al., Antitumor Activity of 1,4-bis(2′-chloroethyl)-1,4-diazabicyclo[2.2.1]heptane dimaleate (Dabis Maleate) In M5076 and Its Subline Resistant to Cyclophosphamide M5/CTX, Annals of Oncology, vol. 3, pp. 233-6 (1992); Maria E. L. van der Burg et al., Phase I Study of DABIS Maleate Given Once Every 3 Weeks, Eur. J. Cancer, vol. 27, pp. 1635-1637 (1991); J. J. M. Holthuis, Etoposide and Teniposide: Bioanalysis, Metabolism and Clinical Pharmacokinetics, Pharmaceutisch Weekblad Sci. Edition, vol. 10, pp. 101-116 (1998); P. Borchmann et al., Phase I Study of BBR 2778, A New Aza-Anthracenedione, In Advanced or Refractory Non-Hodgkin's Lymphoma, Annals of Oncology, vol. 12, pp. 661-667 (2001); J. G. Reves et al., Midazolam Maleate Induction In Patients With Ischaemic Heart Disease: Haemodynamic Observations, Canadian Anesthetists' Soc'y J., vol. 26, no. 5, pp. 402-409 (1979); S. M. Huang et al., Pharmacokinetics of Chlorpheniramine After Intravenous and Oral Administration In Normal Adults, Eur. J. Clinical Pharmacology, vol. 22, pp. 359-365 (1982). A table summary that explains under which circumstances maleic acid is injected into humans is given below (Table 2).
TABLE 2Presence of maleic acid in human injectablesTherapeutic/TotalcompoundClassStatusMA UseDoseMethergineUtertonicApprovedExcipient0.1mgPiritonAntihistamineApprovedSalt form1.2mgBBR2778ChemoPhase IIISalt form2.8mg*DABIS maleateChemoPhase IISalt form34mg*Maleic-2-14CMetabolism studyN/ADirect11mg(1958)
MA is not known to stabilize radiopharmaceuticals. For example, MA was tested as a stabilizer for radiopharmaceutical 99mTc(Sn)-DTPA with a strength of 7-9 mCi/mL. However, in the reported study, MA was “not able to prevent the decomposition of 99mTc(Sn)-DTPA.” Ralf Berger, Radical Scavengers and the Stability of 99mTc-Radiopharmaceuticals, Int'l J. Applied Radiation & Isotopes, vol. 33, pp. 1341-1344 (1982). MA has been used to prevent rancidity in fats for a period of weeks, yet MA's protective effect is diminished in the presence of water. George R. Greenbank & George E. Holm, Antioxidants for Fats and Oils, Indus. & Engineering Chemistry, vol. 26, no. 3, pp. 243-245 (1934).